The goal of this project is to use quantitative light and electron microscopy along with in situ hybridization and immunocytochemistry to study cellular and molecular mechanisms of myelin formation, breakdown and regeneration. This year, morphometric comparisons of sciatic nerve regeneration in young adult and aging mice were continued by supplementing observations 2 weeks after axotomy with findings after 4 weeks and 8 weeks. The results showed that in sciatic nerves of aging mice, regeneration of axons and their myelin sheaths was significantly retarded 2 weeks after axotomy; differences in myelinated axon numbers and sizes were smaller 4 weeks after axotomy and not significantly different 8 weeks later. However, even though the myelinated fiber numbers and their axonal sizes were similar in the two age groups after 8 weeks, the myelin sheath thicknesses and the areas of Schwann cell cytoplasm were significantly less in nerves of old mice at all intervals examined. These morphometric results showed that Schwann cell responses were more affected than axonal responses in aging nerves. Preliminary experiments using conditioning lesions suggested that the brief duration of axon size differences would limit the value of graft experiments in exploring origins of the age-related changes. Other pilot experiments showed that axotomy-induced myelinated fiber breakdown and regeneration in young mice could be delayed by suppression of macophages. Further studies will focus on Schwann cell responses. We also began studying the glial changes and patterns of myelin breakdown in five cases of Balo's concentric sclerosis.